General background materials on fuel rods, claddings and absorber materials are available. See, e.g., Frank J. Rahn et al., A Guide to Nuclear Power Technology, pp. 429-438 (1984).
In light water reactor (LWR) designs, fuel is formed into oxide pellets, which consist of uranium oxide or mixed uranium/plutonium oxide. These pellets are then placed in long tubes called cladding tubes to form fuel rods. The cladding tube forms a barrier against radioactive fission products released in the fuel pellets during irradiation. Proper fuel design requires an economical fuel cycle, while providing the necessary fuel characteristics for safe plant operation. Thus structural materials must be selected that have low neutron cross-section and low cost, while providing adequate mechanical corrosion resistance characteristics. Fuel assembly design should accordingly allow for the operation of the reactor at the design power and for the highest possible burn-up without breaching the cladding and releasing radioactive products to the primary coolant.
Zirconium alloys are used in fuel designs because they combine desirable nuclear, physical and mechanical properties. Because nuclear-grade zirconium is expensive, its alloys are used only in the active zone of the nuclear core where its neutron economy is most advantageous. Zircaloy-2 and Zircaloy-4 are two slightly different alloys which were developed for nuclear applications. Zircaloy-2 typically contains about 1.4 wt. % tin, 0.15 wt. % iron, 0.1 wt. % chromium and 0.06 wt. % nickel, 1,000 ppm oxygen and the balance zirconium. Zircaloy-4 typically contains about 1.4 wt. % tin, 0.21 wt. % iron, 0.11 wt. % chromium, 30 ppm nickel, 1,200 ppm oxygen and the balance zirconium. Zircaloy-2 has a small content of nickel, while in Zircaloy-4 the nickel content is essentially replaced by iron. This small change in composition reduces the hydrogen absorption rate during service in high-temperature water. The physical and mechanical properties of the two alloys are nearly identical.
Pressurized water reactor (PWR) fuel rods are typically made with Zircaloy-4 cladding, while boiling water reactor (BWR) fuel rods utilize Zircaloy-2. However, the channel in a BWR containing the fuel assembly is typically made of Zircaloy-4. Lower and upper tie plates are typically fabricated from type 304 stainless steel. The spacer grids, which keep proper distance between rods along the length of the fuel assembly, are usually made of Zircaloy-4. The end grids, however, are typically made of Inconel-718, chosen for its high corrosion resistance and high strength. Guide thimbles, for the insertion of control rods, are also typically made of Zircaloy-4.
Continuous operation of a reactor requires that the core remain critical. However, to compensate for the gradual depletion of fissile material with time, as burn-up accumulates, and to compensate for other phenomena such as the buildup of fission products, excess reactivity must be built into the nuclear core. This excess reactivity must be controlled at any given time to keep the reactor critical for steady-state operation. This task is accomplished by the use of materials that are strong neutron absorbers or "poisons." Control elements constructed from neutron absorbers regulate power generation according to demand, provide quick shutdown, account for short-term and long-term reactivity changes that result from temperature changes, and adjust for fission product accumulation and fissile material depletion.
The foremost characteristic of a control material is its neutron absorption properties. These vary with the energy of the impinging neutrons but one can gather together the detailed absorption features into a "thermal absorption cross-section," which is of interest in LWR's. The dominant absorber used in control rods in LWR's is boron.
In addition to the movable control rods used in all LWR'S, present LWR designs utilize burnable poisons. These are solid neutron absorbers which are placed in the reactor. As it is subjected to neutron irradiation, the burnable absorber material is gradually depleted. Thus the depletion of the burnable poison corresponds, roughly, to the depletion of fissile material. Burnable-poisons are used to counterbalance excess reactivity at the beginning of the fuel cycle and to provide a means for power shaping and optimum core burn-up. Burnable poison compounds currently of interest include boron, erbium and gadolinium.
Many LWR fuel designs employ burnable absorber rods to control axial power peaking or moderator temperature coefficient in a number of ways. In some designs, burnable absorber rods are placed in fuel assembly lattice locations, thereby displacing fuel rods. Other designs employ burnable absorber rod inserts and fuel assembly guide thimbles. Still other designs involve the formation of burnable-absorber coatings on the inside diameters of cladding tubes, on fuel pellet surfaces, or involve distribution of the burnable absorber within the fuel pellet.
Ideally, it would be desirable to directly include the burnable absorber materials in the reactor structural components to prevent displacement of the nuclear fuel. The use of erbium, boron or gadolinium, however, may tend to compromise the corrosion resistence of the zirconium alloys.
Accordingly, it is a continuing problem in this art to develop an absorber material which can be used as a structural component, while exhibiting adequate corrosion resistance.